Quantum radar is an emerging remote-sensing technology based on input quantum correlations (in particular, quantum entanglement) and output quantum detections. If it is successfully developed, it will allow the radar system to pick out its own signal even when swamped by background noise. This allows it to detect stealth aircraft, filter out deliberate jamming attempts, and operate in areas of high background noise, e.g., due to ground clutter. The first feasible design of a quantum radar was proposed in 2015 by an international team and is based on the protocol of Gaussian quantum illumination.
The basic concept is to create a stream of entangled visible-frequency photons and split it in half. One half, the "signal beam", goes through a conversion to microwave frequencies in a way that preserves the original quantum state. The microwave signal is then sent and received as in a normal radar system. When the reflected signal is received it is converted back into photons and compared with the other half of the original entangled beam, the "idler beam".
Although most of the original entanglement will be lost due to quantum decoherence as the microwaves travel to the target objects and back, enough quantum correlations will still remain between the reflected-signal and the idler beams. Using a suitable quantum detection scheme, the system can pick out just those photons that were originally sent by the radar, completely filtering out any other sources. If the system can be made to work in the field, it represents an enormous advance in detection capability.
One way to defeat conventional radar systems is to broadcast signals on the same frequencies used by the radar, making it impossible for the receiver to distinguish between their own broadcasts and the spoofing signal (or "jamming"). However, such systems cannot know, even in theory, what the original quantum state of the radar's internal signal was. Lacking such information, their broadcasts will not match the original signal and will be filtered out in the correlator. Environmental sources, like ground clutter and aurora, will similarly be filtered out.
There is considerable discussion of the use of quantum radar as an anti-stealth technology. Stealth aircraft are designed to reflect signals away from the radar, typically by using rounded surfaces and avoiding anything that might form a partial corner reflector. This so reduces the amount of signal returned to the radar's receiver that the target is (ideally) lost in the thermal background noise. Although stealth technologies will still be just as effective at reflecting the original signal away from the receiver of a quantum radar, it is the system's ability to separate out the remaining tiny signal, even when swamped by other sources, that allows it to pick out the return even from highly stealthy designs.
The most convincing model was proposed by an international team of researchers. The team designed a model of quantum radar for remote sensing of a low-reflectivity target that is embedded within a bright microwave background, with detection performance well beyond the capability of a classical microwave radar. By using a suitable wavelength "electro-optomechanical converter", this scheme generates excellent quantum entanglement between a microwave signal beam, sent to probe the target region, and an optical idler beam, retained for detection. The microwave return collected from the target region is subsequently converted into an optical beam and then measured jointly with the idler beam. Such a technique extends the powerful protocol of quantum illumination to its more natural spectral domain, namely microwave wavelengths. A prototype quantum radar could be realized with current technology, and is suited to applications from standoff sensing of stealth objects to environmental scanning of electrical circuits. Thanks to its quantum-enhanced sensitivity, this device could lead to low-flux non-invasive techniques for protein spectroscopy and biomedical imaging. Alternative methods were also considered by defense contractor Lockheed Martin whose aim was to create a radar system providing a better resolution and higher detail than classical radar could provide. According to Chinese state media, the first quantum radar was developed and tested by China in real-world environment in August 2016.. More recently, the generation of large numbers of entangled photons for radar detection has been studied by the University of Waterloo.
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